1. Field of the Invention
This invention relates generally to the field of atomic absorption spectrometric analysis, and in particular, to the field of simultaneous multielement atomic absorption spectrometric analysis.
2. Description of Prior Art
In recent years there has been considerable interest in the development of multielement atomic spectrometry systems. Most of these have been based on atomic emission or atomic fluorescence measurement; and in fact it has been said that, compared to these methods, atomic absorption is the least likely candidate for simultaneous multielement development. Nevertheless, atomic absorption has enjoyed wide acceptance and utilization as a trace element analysis tool both by spectroscopists and by scientists whose main interests are in areas other than spectroscopy itself. As a result, the operating principles, atomization devices, sample preparation and introduction procedures, and potential interferences are already familiar to many users. From this point of view it would seem that a multielement atomic absorption spectrometer has some merit after all.
There have been numerous attempts to develop a practical simultaneous multielement atomic absorption spectrometer. Most of these systems suffer from one or more serious limitations, such as the inability to utilize furnace atomizers, limitations in the number of channels, lack of background correction or double beam operation, or complex constructional requirements. Only three systems have been shown to be able to utilize electrothermal atomization devices. One such system (Lundberg and Johansson) used a carbon rod atomizer and a modified monochromator with three exit slits. Only three elements could be determined simultaneously. Background correction was accomplished by using a continuum source in addition to the multielement hollow cathode lamp. A minicomputer was used for data acquisition and processing. Detection limits were a factor of 2 to 4 higher than those obtained when the same atomizer was used with a commercially available atomic absorption spectrometer (AAS) operating in the single element mode. The deterioration of the detection limits was attributed to the time sharing of the data acquisition system, the higher noise and lower intensity of the multielement hollow cathode lamp, and "constructional compromises".
Another such system (Salin and Ingle) developed was similar, having a modified, multiexit slit monochromator. The instrument was restricted to the simultaneous analysis of four elements due to hollow cathode lamp intensity limitations. Reported detection limits were approximately an order of magnitude worse than those reported for commercially available atomic absorption (AA) instruments using electrothermal atomization in the single element mode of operation.
The third system (Alder, et al.) employed a direct reader with the furnace to analyze up to 9 elements simultaneously in the absorption mode. An array of hollow cathode lamps was used as the light source. The direct reader electronics were replaced by a custom-designed analog circuit which generated peak area information. However, there were no provisions for compensating for non-specific background absorption and source fluctuation noise. Detection limits were comparable to commercially available AA instruments using electrothermal atomization.
This invention includes the design, construction, and operation of a simultaneous multielement atomic absorption spectrometer which is based upon a high-intensity continuum primary source, a high-resolution, wavelength modulated direct reading echelle polychromator, and a high-speed computer data acquisition system. This instrument has been dubbed SIMAAC. A single element spectrometer based on the similar optical principles, called CEWM-AA, is known. SIMAAC overcomes many of the limitations of previous designs. As many as 16 or more elements may be measured simultaneously with either flame, furnace or other atomization source. Each channel operates in true double-beam, background corrected mode.
A significant factor in the success of SIMAAC is the use of both a continuum source and wavelength modulation with high speed data acquisition. Prior art multielement AA apparatus have, at times, used a continuum source for sequential multielement analysis. All such apparatus have uniformly yielded unacceptably poor results. Wavelength modulation has been used in single element techniques. Accordingly, those skilled in the art have had no reason to even suspect that the use of a continuum source and wavelength modulation in multielement AA analysis could yield results comparable to single-element analysis. On the contrary, those skilled in the art have believed that utilization of a continuum source and wavelength modulation in multielement AA analysis would inevitably produce poorer results than when either was used individually. With SIMAAC, however, it is possible for the first time to obtain simultaneous multielement data without significant signal-to-noise loss compared to single-element operation.